Remotely operated drones would make more sense for boarding.
Modern composites could bring the weight down a lot.
>Positive and negative mass repel each other.
Not exactly. Positive mass exerts a repulsive force on negative mass, but negative force divided by negative mass equals positive acceleration. The net effect is that negative mass repels positive mass but positive mass attracts negative mass.
Newton says that if a positive mass accelerates in the same direction as an applied force, a negative mass will accelerate in the opposite direction to an applied force.